Lead-free soft solder

ABSTRACT

The invention relates to a soft solder which includes the alloying constituents bismuth and two of the three metals silver, copper and nickel, wherein bismuth forms between 20% by weight and 99.8% by weight of the alloy, silver forms between 0.1% by weight and 50% by weight of the alloy, copper forms between 0.1% by weight and 30% by weight of the alloy and nickel forms between 0.1% by weight and 30% by weight of the alloy.

CROSS REFERENCE TO RELATED APPLICATION

This Utility patent application is a divisional application of U.S.application Ser. No. 10/490,918, filed Dec. 2, 2004, which claims thebenefit of the filing date of Application No. DE 101 47 378.8, filedSep. 26, 2001 and International Application No. PCT/DE02/03396, filedSep. 12, 2002, both of which are herein incorporated by reference.

BACKGROUND

The invention relates to a lead-free soft solder, in particular to anelectronics solder.

Soft solders, in particular lead-tin solders, the melting point of whichis below 330° C., are used in the electronics sector to produce fixedmechanical and electrical connections. Lead-tin solders used on anindustrial scale are standardized, for example, as L-PbSn2 (with a tincontent of 2%) to L-Sn90Pb (with a tin content of 90%). In addition,lead-tin solders with an antimony content of from 0.1% to 5% are alsoused.

Lead-tin solders of this type are also used to solder semiconductorchips in semiconductor fabrication. However, typical alloys have meltingranges which are below the melting point of pure tin (T_(m,Sn)=232° C.).For this reason they are not suitable for applications in whichtemperatures which are higher than the melting range of the solder occurduring operation of the semiconductor component.

However, the thermal demands imposed on electronics and semiconductorcomponents are becoming increasingly severe, since, for example, highlyclocked processors with clock frequencies in the gigahertz range canreach very high operating temperatures. This means that in somelocations at module level, soldering is no longer carried out usingeutectic Pb-62Sn solder (with a melting point of T_(m,Pb-62Sn)=183° C.),but rather silver-containing tin solders which do not contain any leadand therefore melt at high temperatures, for example using tin-silver(SnAg; T_(m,SnAg)=221° C.) or using tin-silver-copper (SnAgCu;T_(m,SnAgCu)=217° C.).

If components are to be processed using these solders, the solderinglocations have to be heated to higher temperatures to enable the solderto be reliably liquefied. For an optimum soldering result, a processingtemperature of 15 to 30° C. above the melting point of the solder isrequired. Where components were soldered to system module carriers andprinted circuit boards using lead-containing solders at temperaturesaround 225° C., said lead-free solders have to be processed attemperatures of 260° C. or above.

In order not to cause any damage during processing, the electronics andsemiconductor components to be soldered have to be able to withstandtemperatures of this level. A temperature limit for the solderingprocessing of semiconductor chips is 420° C., since most semiconductorsare destroyed above this temperature. Many components, for example thoseused in plastic ball grid arrays or other plastic casings, however, arealready damaged even at temperatures much lower than 420° C.

Soldered connections which use solder with a relatively high leadcontent, for example Pb-5Sn-2Ag (T_(m,Pb-5Sn-2Ag)=288° C.) are used aschip solder to produce soldered connections which are able to withstandthe high operating temperatures which occur and which do not cause anydamage to the components during processing. However, this alloy has apronounced ductility and can lead to mechanical failure of the chipsoldered joint after only a relatively short operating time.

A further problem with processing lead-containing electronics soldersresides in their long-term toxicity, since lead, as a toxic heavy metal,has the ability to build up in living organisms, where it can causelong-term harm. In particular during the processing, i.e., liquefactionof the lead-containing solder, lead vapors are released and have to befiltered out at high cost. However, the lead-containing solders alsocause extensive problems during disposal and recycling of electronicsscrap.

SUMMARY

One embodiment of the present invention provides an electronics solderthat can satisfy future requirements with regard to thermal stabilityand environmental compatibility.

In a first variant of the invention, a soft solder that is suitable inparticular for use in the electronics sector includes the alloyingconstituent bismuth (Bi), silver (Ag) and copper (Cu). According to thisfirst variant of the invention, bismuth forms between 20 percent byweight (% by weight) and 99.8 percent by weight of the alloy, silverforms between 0.1% by weight and 50% by weight of the alloy, and copperforms between 0.1% by weight and 30% by weight of the alloy.

This soft solder according to one embodiment of the invention comprisingthe alloying constituent bismuth, silver and copper does not contain anylead and therefore represents an environmentally friendly solder.Moreover, it has a high mechanical strength and can therefore ensure areliable connection between metallic surfaces. On account of the factthat the lowest melting point of this soft solder is 258° C., it issuitable for processing temperatures below 420° C. and for joints whichmust not melt even at operating temperatures of up to 260° C.Consequently, this soft solder can be used to solder together evenmetals which hitherto had to be soldered using environmentally damagingsolders with a high lead content, since only the latter was suitable forrelatively high melting points.

With this solder according to one embodiment of the invention, thesoldered connection between liquid alloy and the metal parts to besoldered is no longer produced by tin, as has hitherto been the case,but rather by the main component bismuth. The use of bismuth as the mainconstituent opens up the availability of higher melting temperatures andmakes the soldered connection more reliable. The soft solder can beprocessed using the conventional processes.

According to one embodiment of the invention, bismuth forms between 63%by weight and 77% by weight of the alloy, silver forms between 20 and30% by weight of the alloy and copper forms between 0.1% by weight and10% by weight of the alloy. In another embodiment bismuth forms between68 and 72% by weight of the alloy, silver forms between 24 and 26% byweight of the alloy, and copper forms between 4.8 and 5.2% by weight ofthe alloy. According to another embodiment of the soft solder accordingto the invention, bismuth forms approximately 70% by weight of thealloy, silver forms approximately 25% by weight of the alloy and copperforms approximately 5% by weight of the alloy. In this case, thetolerance range for bismuth and silver in the alloy is in each case ±2%by weight. The tolerance range for copper in the alloy is ±1% by weight.This bismuth-based soft solder (Bi52-Ag36-Cul2, atomic percent) has asolidus temperature of 261° C. and a liquidus temperature ofapproximately 400° C. and is therefore suitable for solderingelectronics components which are subject to high thermal loads, reachingoperating temperatures of well over 260° C.

According to another embodiment of the soft solder according to theinvention, bismuth forms between 78% by weight and 92% by weight of thealloy, silver forms between 0.1% by weight and 20% by weight of thealloy, and copper forms between 0.1% by weight and 10% by weight of thealloy. In another, bismuth forms between 83 and 87% by weight of thealloy, silver forms between 9 and 11% by weight of the alloy, and copperforms between 4.8 and 5.2% by weight of the alloy. In anotherembodiment, bismuth forms approximately 85% by weight (±2% by weight) ofthe alloy, silver forms approximately 10% by weight (±2% by weight) ofthe alloy, and copper forms approximately 5% by weight (±1% by weight)of the alloy. On account of its lower silver content, the soft soldercomprising the alloying constituents in these proportions is inexpensiveand is likewise suitable for applications in relatively high temperatureranges, since it has a solidus temperature of 260° C. and a liquidustemperature of approximately 350° C. This soft solder alloy can bedescribed as a Bi70-Ag16-Cu14 (in atomic percent).

According to another embodiment of the soft solder, bismuth formsbetween 83% by weight and 97% by weight of the alloy, silver formsbetween 0.1 and 20% by weight of the alloy, and copper forms between 0.1and 1% by weight of the alloy. In another, bismuth forms between 88 and92% by weight of the alloy, silver forms between 9 and 11% by weight ofthe alloy and copper forms between 0.1 and 0.5% by weight of the alloy.In this context, it is particularly preferable for bismuth to formapproximately 90% by weight (±2% by weight) of the alloy, for silver toform approximately 10% by weight (±2% by weight) of the alloy and forcopper to form approximately 0.1% by weight (±0.11% by weight) of thealloy. This very low-copper soft solder can be referred to asWi82-Ag18-Cu0.3 (in atomic percent) and has a solidus temperature of261° C. and a liquidus temperature of approximately 350° C.

According to another form of the soft solder according to the invention,bismuth forms between 83 and 97% by weight of the alloy, silver formsbetween 0.1 and 10% by weight of the alloy, and copper forms between 0.1and 10% by weight of the alloy. In another, bismuth forms between 88 and92% by weight of the alloy, silver forms between 4.8 and 5.2% by weightof the alloy, and copper forms between 4.8 and 5.2% by weight of thealloy. According to another form, bismuth forms approximately 90% byweight (±2% by weight) of the alloy, silver forms approximately 5% byweight (±2% by weight) of the alloy, and copper forms approximately 5%by weight (±1% by weight) of the alloy. This soft solder, which can bedescribed as Wi78-Ag8-Cu14 (in atomic percent), has a solidustemperature of 261° C. and a liquidus temperature of approximately 350°C.

Furthermore, another variant of the invention provides a soft solder, inparticular an electronics solder, that includes the alloyingconstituents bismuth, copper and nickel, with bismuth forming between40% by weight and 99.8% by weight of the alloy, copper forming between0.1% by weight and 20% by weight of the alloy and nickel forming between0.1% by weight and 20% by weight of the alloy. This soft solderaccording to one embodiment of the invention is particularly suitablefor processing temperatures below 420° C. and for joints which must notmelt at temperatures up to 260° C., since the solidus temperature is atleast 266° C. and the liquidus temperature is between 620° C. and 850°C.

According to another embodiment of the soft solder according to theinvention, bismuth forms between 71 and 85% by weight of the alloy,copper forms between 15 and 25% by weight of the alloy, and nickel formsbetween 0.1 and 5% by weight of the alloy. In another embodiment of theinvention bismuth forms between 76 and 80% by weight of the alloy,copper forms between 18.5 and 21.5% by weight of the alloy and nickelforms between 1.8 and 2.2% by weight of the alloy. In another embodimentof the soft solder, bismuth forms approximately 78% by weight (±2% byweight) of the alloy, copper forms approximately 20% by weight (±2% byweight) of the alloy, and nickel forms approximately 2% by weight (±1%by weight) of the alloy. This soft solder can be described asBi52-Cu43-Ni5 (in atomic percent) and has a solidus temperature ofapproximately 266° C. and a liquidus temperature of approximately 830 to850° C.

According to another embodiment of the soft solder according to theinvention, bismuth forms between 82 and 96% by weight of the alloy,copper forms between 5 and 15% by weight of the alloy, and nickel formsbetween 0.1 and 3% by weight of the alloy. In another, bismuth formsbetween 87 and 91% by weight of the alloy, copper forms between 9 and11% by weight of the alloy and nickel forms between 0.8 and 1.2% byweight of the alloy. In another embodiment, bismuth forms approximately89% by weight (±2% by weight) of the alloy, copper forms approximately10% by weight (±2% by weight) of the alloy, and nickel formsapproximately 1% by weight (±±0.5% by weight) of the alloy. This softsolder can be described as Bi71-Cu26-Ni3 (in atomic percent) and has asolidus temperature of 266° C. and a liquidus temperature of 720 to 740°C.

According to another embodiment of the soft solder according to theinvention, bismuth forms between 78 and 99.8% by weight of the alloy,copper forms between 2 and 8% by weight of the alloy, and nickel formsbetween 0.1 and 3% by weight of the alloy. In one embodiment, bismuthforms between 92 and 96% by weight of the alloy, copper forms between 4and 6% by weight of the alloy, and nickel forms between 0.8 and 1.2% byweight of the alloy.

According to one embodiment, bismuth forms approximately 94% by weight(+2% by weight) of the alloy, copper forms approximately 5% by weight(±2% by weight) of the alloy, and nickel forms approximately 1% byweight (±0.5% by weight) of the alloy. This soft solder can becharacterized as Bi82-Cu15-Ni3 (in atomic percent) and has a solidustemperature of 266° C. and a liquidus temperature of between 620 and660° C.

According to another embodiment of the soft solder, bismuth formsbetween 88 and 99.8% by weight of the alloy, copper forms between 2 and8% by weight of the alloy, and nickel forms approximately 0.1% by weightof the alloy. In one embodiment, bismuth forms between 93 and 97% byweight of the alloy, for copper forms between 4 and 6% by weight of thealloy, and nickel forms approximately 0.1% by weight of the alloy.According to one embodiment, bismuth forms approximately 95% by weight(±2% by weight) of the alloy, and copper forms approximately 5% byweight (±2% by weight) of the alloy. The nickel in each case forms 0.1%by weight (±0.05% by weight) of the alloy. This soft solder, which canbe described as Bi85-Cu15-Ni0.3 (in atomic percent), has a solidustemperature of 266° C. and a liquidus temperature of 620 to 660° C.

According to another embodiment, a further soft solder according to theinvention includes the alloying constituents bismuth, silver and nickel,with bismuth forming between 20% by weight and 99.8% by weight of thealloy, silver forming between 0.1% by weight and 50% by weight of thealloy and nickel forming between 0.1% by weight and 30% by weight of thealloy. This bismuth-silver-nickel solder is suitable for processingtemperatures below 420° C. and for joints which must not melt attemperatures of up to 260° C. The solidus temperature of this softsolder is approximately 260° C. and the liquidus temperature between360° C. and 430° C., depending on the mixing ratio of the alloyingconstituents.

According to another embodiment of the bismuth-silver-nickel solder,bismuth forms between 61 and 75% by weight of the alloy, silver formsbetween 25 and 35% by weight of the alloy, and nickel forms between 0.1and 5% by weight of the alloy. In one embodiment, bismuth forms between66 and 70% by weight of the alloy, silver forms between 28 and 32% byweight of the alloy and nickel forms between 1.8 and 2.2% by weight ofthe alloy. According to one embodiment bismuth forms approximately 68%by weight of the alloy, silver forms approximately 30% by weight of thealloy, and nickel forms approximately 2% by weight of the alloy. ThisBi51-Ag44-Ni5 (in atomic percent) has a solidus temperature of 260° C.and a liquidus temperature of 430° C.

According to another embodiment of the soft solder, bismuth formsbetween 71 and 85% by weight of the alloy, silver forms between 15 and25% by weight of the alloy, and nickel forms between 0.1 and 5% byweight of the alloy. In one embodiment, bismuth forms between 76 and 80%by weight of the alloy, silver forms between 19 and 21% by weight of thealloy and nickel forms between 1.8 and 2.2% by weight of the alloy.According to one embodiment of the invention, bismuth formsapproximately 78% by weight of the alloy, silver forms approximately 20%by weight of the alloy and nickel forms approximately 2% by weight ofthe alloy. This soft solder can be referred to as Bi63-Ag31-Ni6 (inatomic percent) and has a solidus temperature of 260° C. and a liquidustemperature of 410° C.

According to another embodiment of the soft solder, bismuth formsbetween 81 and 95% by weight of the alloy, silver forms between 5 and15% by weight of the alloy and nickel forms between 0.1 and 5% by weightof the alloy. In one embodiment, bismuth forms between 86 and 90% byweight of the alloy, silver forms between 8 and 12% by weight of thealloy, and nickel forms between 1.8 and 2.2% by weight of the alloy.According to one embodiment, bismuth forms approximately 88% by weightof the alloy, silver forms approximately 10% by weight of the alloy, andnickel forms approximately 2% by weight of the alloy. This soft solder,which can be referred to as Bi77-Ag17-Ni6, has a solidus temperature of260° C. and a liquidus temperature of 380° C.

According to another embodiment of the soft solder, bismuth formsbetween 82 and 97% by weight of the alloy, silver forms between 5 and15% by weight of the alloy, and nickel forms approximately 0.1% byweight of the alloy. In one embodiment, bismuth forms between 88 and 92%by weight of the alloy, silver forms between 8 and 12% by weight of thealloy and nickel forms approximately 0.1% by weight of the alloy.According to one embodiment of the soft solder, bismuth formsapproximately 90% by weight of the alloy, silver forms approximately 10%by weight of the alloy and nickel forms approximately 0.1% by weight ofthe alloy. This soft solder, which can be referred to as Bi82-Ag18-Ni0.3(in atomic percent), has a solidus temperature of 260° C. and a liquidustemperature of 360° C.

In all these variants of the soft solder which have been mentioned(Bi—Ag—Cu, Bi—Cu—Ni and Bi—Ag—Ni), bismuth in each case represents thebinding main component, so that higher processing temperatures can beachieved compared to conventional tin solders. Moreover, these bismuthsolders represent an environmentally friendly alternative with a veryhigh reliability in operation. All the alloys which have been mentionedin accordance with the invention can be processed without problems usingexisting processing machines.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles of the invention. Other embodiments ofthe present invention and many of the intended advantages of the presentinvention will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 illustrates a binary phase diagram for bismuth and copper.

FIG. 2 illustrates a binary phase diagram for silver and bismuth.

FIG. 3 illustrates a ternary phase diagram for silver, bismuth andcopper.

FIG. 4 illustrates a further binary phase diagram for bismuth andcopper.

FIG. 5 illustrates a binary phase diagram for bismuth and nickel.

FIG. 6 illustrates a ternary phase diagram for bismuth, copper andnickel.

FIG. 7 illustrates a further binary phase diagram for bismuth andsilver.

DETAILED DESCRIPTION

FIG. 1 illustrates a binary phase or equilibrium diagram for bismuth andcopper, in which decreasing percentages by weight of copper (Cu) from100% by weight on the left to 0% by weight on the right are plotted on alower horizontal axis. Accordingly, increasing percentages by weight ofbismuth (Bi) from 100% by weight on the right to 0% by weight on theleft are plotted on the same axis. Atomic percentages of bismuth whichincrease from 0 atomic % on the left to 100 atomic % on the right areplotted on an upper horizontal axis. Temperatures from 200° C. to 1200°C. are plotted on a vertical axis of the diagram.

The diagram illustrates a solidus area below a first solidus line 2, inwhich both copper and bismuth are in the form of pure crystals. Thisstate of affairs is illustrated in the diagram by the followingindication:

(Cu)+(Bi),

where the element symbols are in each case in parentheses (cf. the smallexcerpt within the diagram). This range lies below a temperature of270.6° C.

A partial melt, in which individual copper crystals (Cu) are dispersedin a bismuth melt—characterized here as L—is present between the firstsolidus line 2 and a first liquidus line 4. This relationship isrepresented by the indication

(Cu)+L.

As can be seen from the point of intersection between the first liquidusline 4 and the left-hand vertical axis of the diagram, pure copper has asolidification point of 1084.87° C. As the proportion of bismuth in thealloy increases, this melting point drops to a value of 270.6° C. wherebismuth forms 99.8% by weight of the alloy (cf. small figure in thecenter). This alloy composition is illustrated by the point ofintersection between the first liquidus line 4 and the first solidusline 2 right by the right-hand vertical axis.

Above the first liquidus line 4, the Bi—Cu alloy is in the form of apure melt without any individual bismuth or copper crystals. This areais characterized by an “L” in the diagram.

FIG. 2 illustrates a corresponding binary phase diagram for silver andbismuth, in which decreasing percentages by weight of silver (Ag) from100% by weight on the left to 0% by weight on the right are plotted on alower horizontal axis of the diagram. Accordingly, decreasingpercentages by weight of bismuth (Bi) from 100% by weight on the rightto 0% by weight on the left are plotted on the same axis. Increasingatomic percentages of bismuth from 0 atomic % on the left to 100 atomic% on the right are plotted on an upper horizontal axis. Temperaturesfrom 100° C. to 1100° C. are plotted on a vertical axis of the diagram.

The diagram illustrates a solidus area below a second solidus line 6, inwhich both silver and bismuth are in the form of pure crystals. Thisstate of affairs is illustrated in the diagram by the followingindication:

(Ag)+(Bi),

where the element symbols are in each case in parentheses. This arealies below a temperature of 262.5° C.

A partial melt, in which individual silver crystals (Ag) are dispersedin a bismuth melt, characterized here as L, is present between thesecond solidus line 6 and a second liquidus line 8. This relationship isillustrated by the indication:

(Ag)+L.

As can be seen from the point of intersection between the secondliquidus line 8 and the left-hand vertical axis of the diagram, puresilver has a solidification point of 961.93° C. As the bismuth contentin the alloy increases, this melting point drops to a value of 262.5° C.where bismuth forms 97.5% by weight of the alloy. This alloy compositionis represented by the point of intersection between the second liquidusline 8 and the second solidus line 6 right at the right-hand verticalaxis.

Moreover, silver has the property of occurring at what is known as aα-solid solution at high concentrations and at temperatures aboveapproximately 170° C. up to its melting point of approximately 961° C.,as illustrated by an area to the left of a slightly convex curve 10 inthe diagram.

Above the second liquidus line 8, the bismuth-silver alloy is in theform of a pure melt, which does not contain any individual bismuth orsilver crystals, and therefore the metals are completely dissolved. Thisarea is characterized by an “L” in the diagram.

FIG. 3 illustrates a ternary phase diagram for silver, bismuth andcopper, in which atomic percentages between 0 and 100 for the threecomponents are plotted on the three axes of the triangle. Decreasingcopper concentrations from 100 atomic percent of Cu on the right to 0atomic percent of Cu on the left are plotted on the horizontal axis.Decreasing silver concentrations from 100 atomic % of Ag at the bottomto 0 atomic % of Ag at the top are plotted on the left-hand axis slopingup to the right. Decreasing bismuth concentrations from 100 atomic % ofBi at the top to 0 atomic % of Bi at the bottom are plotted on theright-hand axis which slopes downward to the left.

Temperature lines are plotted within the three axes. A pronounced thirdliquidus line 12, at which the temperature lines each have a pronouncedkink, can be seen in the left-hand half of the triangle enclosed by thethree axes.

Four different points A1, B1, C1 and D1 are plotted in this diagram,characterizing the four alloy compositions compiled in the table below.

Solidus Liquidus temperature temperature Alloys [° C.] [° C.] A1:Bi52—Ag36—Cu12 261 Approximately 400 [atomic %] B1: Bi70—Ag16—Cu14 261Approximately 350 [atomic %] C1: Bi82—Ag18—Cu0.3 261 Approximately 350[atomic %] D1: Bi78—Ag8—Cu14 261 Approximately 350 [atomic %]

Alloy A1 (Bi52-Ag36-Cu12) contains approximately 70% by weight (±2% byweight) of bismuth, approximately 25% by weight (±2% by weight) ofsilver and approximately 5% by weight (±1% by weight) of copper. AlloyBi (Bi70-Ag16-Cu14) contains approximately 85% by weight (±2% by weight)of bismuth, approximately 10% by weight (±2% by weight) of silver andapproximately 5% by weight (±1% by weight) of copper. Alloy C1(Bi82-Ag18-Cu0.3) contains approximately 90% by weight (±2% by weight)of bismuth, approximately 10% by weight (±2% by weight) of silver andapproximately 0.1% by weight (±0.1% by weight) of copper. Alloy D1(Bi78-Ag8-Cu14) contains approximately 90% by weight (±2% by weight) ofbismuth, approximately 5% by weight (±2% by weight) of silver andapproximately 5% by weight (±1% by weight) of copper.

The plotted alloys A1 to D1 can if appropriate be understood as widerranges which may therefore adopt a larger area than that illustrated inthe diagram.

An enlarged excerpt from the apex of the triangle illustrated by thediagram illustrates a point E at which the three components incrystalline form at a temperature of approximately 258° C. are in eachcase in equilibrium with the melt L, as indicated by the followingreaction scheme

E:L

(Ag)+(Bi)+(Cu).

FIG. 4 illustrates a binary phase diagram for bismuth and coppercorresponding to that shown in FIG. 1, but in this case preferred alloycompositions A2, B2, C2 and D2 in accordance with the following tableare also included in the drawing.

Solidus Liquidus temperature temperature Alloys [° C.] [° C.] A2:Bi52—Cu43—Ni5 266 830 . . . 850 [atomic %] B2: Bi71—Cu26—Ni3 266 720 . .. 740 [atomic %] C2: Bi82—Cu15—Ni3 266 620 . . . 660 [atomic %] D2:Bi85—Cu15—Ni0.3 266 620 . . . 660 [atomic %]

Alloy A2 (Bi52-Cu43-Ni5) contains approximately 78% by weight (±2% byweight) of bismuth, approximately 20% by weight (±2% by weight) ofcopper and approximately 2% by weight (±1% by weight) of nickel. AlloyB2 (Bi71-Cu26-Ni3) contains approximately 89% by weight (±2% by weight)of bismuth, approximately 10% by weight (±2% by weight) of copper andapproximately 1% by weight (±0.5% by weight) of nickel. Alloy C2(Bi82-Cu15-Ni3) contains approximately 94% by weight (±2% by weight) ofbismuth, approximately 5% by weight (±2% by weight) of copper andapproximately 1% by weight (±0.5% by weight) of nickel. Alloy D2(Bi85-Cu15-Ni0.3) contains approximately 95% by weight (±2% by weight)of bismuth, approximately 5% by weight (±2% by weight) of copper andapproximately 0.1% by weight (±0.05% by weight) of nickel.

Furthermore, FIG. 5 illustrates a binary phase diagram for bismuth andnickel, in which decreasing percentages by weight of nickel (Ni) from100% by weight on the left to 0% by weight on the right are plotted on alower horizontal axis of the diagram. Accordingly, decreasingpercentages by weight of bismuth (Bi) from 100% by weight on the rightto 0% by weight on the left are plotted on the same axis. Increasingatomic percentages of bismuth from 0 atomic % on the left to 100 atomic% on the right are plotted on an upper horizontal axis. Temperaturesfrom 200° C. to 1600° C. are plotted on a vertical axis of the diagram.

The diagram illustrates a solidus area below a third solidus line 14, inwhich both nickel and bismuth are in the form of pure crystals. Thisstate of affairs is illustrated in the diagram by the followingindication:

(Ni)+(Bi),

where the element symbols are in each case in parentheses. This solidusarea lies below a temperature of 654° C.

A partial melt, in which individual nickel crystals (Ni) are dispersedin a bismuth melt, characterized here as L, is present between the thirdsolidus line 14 and a fourth liquidus line 16. This relationship isillustrated by the indication:

(Ni)+L.

As can be seen from the point of intersection between the fourthliquidus line 16 and the left-hand vertical axis of the diagram, purenickel has a solidification point of 1455° C. As the proportion ofbismuth in the alloy increases, this melting point decreases in a numberof stages to less than 200° C. where bismuth forms 99.8% by weight ofthe alloy.

In a range where bismuth forms between approximately 74% by weight and77% by weight, at temperatures below 654° C. an intermetallic phase NiBiis formed, which tapers to a point below and toward the third solidusline 14. Therefore, at temperatures just below 654° C., theintermetallic phase is only established with a very precise ratio ofnickel to bismuth.

To the right of the intermetallic phase NiBi there is a eutectic 18 at atemperature of 469° C., characterized by a horizontal line. A furtherintermetallic phase NiBi₃ is formed at a fixed mixing ratio ofapproximately 90% by weight of Bi below the eutectic 18, i.e. attemperatures of less than 469° C.

FIG. 6 illustrates a ternary phase diagram for bismuth, copper andnickel, in which atomic percentages of between 0 and 100 for the threecomponents are plotted on the respective three axes of the triangle.Decreasing concentrations of nickel from 100 atomic % of Ni on the rightto 0 atomic % of Ni on the left are plotted on the horizontal axis.Decreasing concentrations of bismuth from 100 atomic % of Bi at thebottom to 0 atomic % of Bi at the top are plotted on the left-hand axis,which slopes up and to the right. Decreasing concentrations of copperfrom 100 atomic % of Cu at the top to 0 atomic % of Cu at the bottom areplotted on the right-hand axis which slopes down and to the left.

Temperature lines are visible within the three axes. In this case, thereis no solidus line as in FIG. 3, and therefore continuous temperaturecurves are plotted.

An interrupted curve which characterizes various transition statesbetween the pure crystals, the intermetallic phases which occur and themelt, is illustrated in a bottom left-hand corner area of the diagram:

E:L

(Bi)+Bi₃Ni+(Cu,Ni)

U:L+BiNi

Bi₃Ni+(Cu,Ni)

Finally, FIG. 7 illustrates a further binary phase diagram for silverand bismuth corresponding to that shown in FIG. 2, but in this casepreferred alloy compositions A3, B3, C3 and D3 in accordance with thefollowing table are also included in the drawing.

Solidus Liquidus temperature temperature Alloys [° C.] [° C.] A3:Bi51—Ag44—Ni5 260 430 [atomic %] B3: Bi63—Ag31—Ni6 260 410 [atomic %]C3: Bi77—Ag17—Ni6 260 380 [atomic %] D3: Bi82—Ag18—Ni0.3 260 360 [atomic%]

Alloy A3 (Bi51-Ag44-Ni5) contains approximately 68% by weight (±2% byweight) of bismuth, approximately 30% by weight (±2% by weight) ofsilver and approximately 2% by weight (±1% by weight) of nickel. AlloyB3 (Bi63-Ag31-Ni6) contains approximately 78% by weight (±2% by weight)of bismuth, approximately 20% by weight (±2% by weight) of silver andapproximately 2% by weight (±1% by weight) of nickel. Alloy C3(Bi77-Ag17-Ni6) contains approximately 88% by weight (±2% by weight) ofbismuth, approximately 10% by weight (±2% by weight) of silver andapproximately 2% by weight (±1% by weight) of nickel. Alloy D3(Bi82-Ag18-Ni0.3) contains approximately 90% by weight (±2% by weight)of bismuth, approximately 10% by weight (±2% by weight) of silver andapproximately 0.1% by weight (±0.01% by weight) of nickel.

The plotted alloys A1 to D1 plotted can if appropriate be understood aswider ranges which may therefore adopt a larger area than thatillustrated in the diagram.

1. A soft solder, in particular an electronics solder, comprising:alloying constituents bismuth, silver and copper, wherein bismuth formsbetween about 20% by weight and about 99.8% by weight of the alloy,silver forms between about 0.1% by weight and about 50% by weight of thealloy, and copper forms between about 0.1% by weight and about 30% byweight of the alloy.
 2. The soft solder of claim 1, wherein bismuthforms between about 63% by weight and about 77% by weight of the alloy,silver forms between about 20% by weight and about 30% by weight of thealloy, and copper forms between about 0.1% by weight and about 10% byweight of the alloy.
 3. The soft solder of claim 1, wherein bismuthforms between about 68% by weight and about 72% by weight of the alloy,silver forms between about 24% by weight and about 26% by weight of thealloy, and copper forms between about 4.8% by weight and about 5.2% byweight of the alloy.
 4. The soft solder of claim 1, wherein bismuthforms about 70% by weight of the alloy, silver forms about 25% by weightof the alloy and copper forms about 5% by weight of the alloy.
 5. Thesoft solder of claim 1, wherein bismuth forms between about 78% byweight and about 92% by weight of the alloy, silver forms between about0.1% by weight and about 20% by weight of the alloy, and copper formsbetween about 0.1% by weight and about 10% by weight of the alloy. 6.The soft solder of claim 1, wherein bismuth forms between about 83% byweight and about 87% by weight of the alloy, silver forms between about9% by weight and about 11% by weight of the alloy and copper formsbetween about 4.8% by weight and about 5.2% by weight of the alloy. 7.The soft solder of claim 1, wherein bismuth forms about 85% by weight ofthe alloy, silver forms about 10% by weight of the alloy and copperforms about 5% by weight of the alloy.
 8. The soft solder of claim 1,wherein bismuth forms between about 88% by weight and about 92% byweight of the alloy, silver forms between about 9% by weight and about11% by weight of the alloy, and copper forms between about 0.1% byweight and about 0.5% by weight of the alloy.
 9. The soft solder ofclaim 1, wherein bismuth forms about 90% by weight of the alloy, silverforms about 10% by weight of the alloy and copper forms about 0.1% byweight of the alloy.
 10. The soft solder of claim 1, wherein bismuthforms between about 88% by weight and about 92% by weight of the alloy,silver forms between about 4.8% by weight and about 5.2% by weight ofthe alloy, and copper forms between about 4.8% by weight and about 5.2%by weight of the alloy.
 11. The soft solder of claim 1, wherein bismuthforms about 90% by weight of the alloy, silver forms about 5% by weightof the alloy and copper forms about 5% by weight of the alloy.
 12. Asoft solder, in particular an electronics solder, comprising: alloyingconstituents bismuth, copper and nickel, wherein bismuth forms betweenabout 40% by weight and about 99.8% by weight of the alloy, copper formsbetween about 0.1% by weight and about 20% by weight of the alloy andnickel forms between about 0.1% by weight and about 20% by weight of thealloy.
 13. The soft solder of claim 12, wherein bismuth forms betweenabout 71% by weight and about 85% by weight of the alloy, copper formsbetween about 15% by weight and about 25% by weight of the alloy andnickel forms between about 0.1% by weight and about 5% by weight of thealloy.
 14. The soft solder of claim 12, wherein bismuth forms betweenabout 76% by weight and about 80% by weight of the alloy, copper formsbetween about 18.5% by weight and about 21.5% by weight of the alloy andnickel forms between about 1.8% by weight and about 2.2% by weight ofthe alloy.
 15. The soft solder of claim 12, wherein bismuth forms about78% by weight of the alloy, copper forms about 20% by weight of thealloy and nickel forms about 2% by weight of the alloy.
 16. The softsolder of claim 12, wherein bismuth forms between about 82% by weightand about 96% by weight of the alloy, copper forms between about 5% byweight and about 15% by weight of the alloy and nickel forms betweenabout 0.1% by weight and about 3% by weight of the alloy.
 17. The softsolder of claim 12, wherein bismuth forms between about 87% by weightand about 91% by weight of the alloy, copper forms between about 9% byweight and about 11% by weight of the alloy and nickel forms betweenabout 0.8% by weight and about 1.2% by weight of the alloy.
 18. The softsolder of claim 12, wherein bismuth forms about 89% by weight of thealloy, copper forms about 10% by weight of the alloy and nickel formsabout 1% by weight of the alloy.
 19. The soft solder of claim 12,wherein bismuth forms between about 87% by weight and about 99.8% byweight of the alloy, copper forms between about 2% by weight and about8% by weight of the alloy and nickel forms between about 0.1% by weightand about 3% by weight of the alloy.
 20. The soft solder of claim 12,wherein bismuth forms between about 92% by weight and about 96% byweight of the alloy, copper forms between about 4% by weight and about6% by weight of the alloy and nickel forms between about 0.8% by weightand about 1.2% by weight of the alloy.
 21. The soft solder of claim 12,wherein bismuth forms about 94% by weight of the alloy, copper formsabout 5% by weight of the alloy and nickel forms about 1% by weight ofthe alloy.
 22. The soft solder of claim 12, wherein bismuth formsbetween about 88% by weight and about 99.8% by weight of the alloy,copper forms between about 2% by weight and about 8% by weight of thealloy and nickel forms about 0.1% by weight of the alloy.
 23. The softsolder of claim 12, wherein bismuth forms between about 93% by weightand about 97% by weight of the alloy, copper forms between about 4% byweight and about 6% by weight of the alloy and nickel forms about 0.1%by weight of the alloy.
 24. The soft solder of claim 12, wherein bismuthforms about 95% by weight of the alloy, copper forms about 5% by weightof the alloy and nickel forms about 0.1% by weight of the alloy.
 25. Asoft solder, in particular an electronics solder, comprising: alloyingconstituents bismuth, silver and nickel, wherein bismuth forms betweenabout 20% by weight and about 99.8% by weight of the alloy, silver formsbetween about 0.1% by weight and about 50% by weight of the alloy andnickel forms between about 0.1% by weight and about 30% by weight of thealloy.
 26. The soft solder of claim 25, wherein bismuth forms betweenabout 61% by weight and about 75% by weight of the alloy, silver formsbetween about 25% by weight and about 35% by weight of the alloy andnickel forms between about 0.1% by weight and about 5% by weight of thealloy.
 27. The soft solder of claim 25, wherein bismuth forms betweenabout 66% by weight and about 70% by weight of the alloy, silver formsbetween about 28% by weight and about 32% by weight of the alloy andnickel forms between about 1.8% by weight and about 2.2% by weight ofthe alloy.
 28. The soft solder of claim 25, wherein bismuth forms about68% by weight of the alloy, silver forms about 30% by weight of thealloy and nickel forms about 2% by weight of the alloy.
 29. The softsolder of claim 25, wherein bismuth forms between about 71% by weightand about 85% by weight of the alloy, silver forms between about 15% byweight and about 25% by weight of the alloy and nickel forms betweenabout 0.1% by weight and about 5% by weight of the alloy.
 30. The softsolder of claim 25, wherein bismuth forms between about 76% by weightand about 80% by weight of the alloy, silver forms between about 19% byweight and about 21% by weight of the alloy and nickel forms betweenabout 1.8% by weight and about 2.2% by weight of the alloy.
 31. The softsolder of claim 25, wherein bismuth forms about 78% by weight of thealloy, silver forms about 20% by weight of the alloy and nickel formsabout 2% by weight of the alloy.
 32. The soft solder of claim 25,wherein bismuth forms between about 81% by weight and about 95% byweight of the alloy, silver forms between about 5% by weight and about15% by weight of the alloy and nickel forms between about 0.1% by weightand 5% by weight of the alloy.
 33. The soft solder of claim 257, whereinbismuth forms between about 86% by weight and about 90% by weight of thealloy, silver forms between about 8% by weight and about 12% by weightof the alloy, and nickel forms between about 1.8% by weight and about2.2% by weight of the alloy.
 34. The soft solder of claim 25, whereinbismuth forms about 88% by weight of the alloy, silver forms about 10%by weight of the alloy and nickel forms about 2% by weight of the alloy.35. The soft solder of claim 25, wherein bismuth forms between about 82%by weight about 97% by weight of the alloy, silver forms between about5% by weight and about 15% by weight of the alloy and nickel forms about0.1% by weight of the alloy.
 36. The soft solder of claim 25, whereinbismuth forms between about 88% by weight and about 92% by weight of thealloy, silver forms between about 8% by weight and about 12% by weightof the alloy, and nickel forms about 0.1% by weight of the alloy. 37.The soft solder of claim 25, wherein bismuth forms about 90% by weightof the alloy, silver forms about 10% by weight of the alloy and nickelforms about 0.1% by weight of the alloy.